CVE-2021-34767: Cisco IOS XE

High severity, CVSS 7.4. EPSS: 0.8% chance of exploitation in the next 30 days.

A vulnerability in IPv6 traffic processing of Cisco IOS XE Wireless Controller Software for Cisco Catalyst 9000 Family Wireless Controllers could allow an unauthenticated, adjacent attacker to cause a Layer 2 (L2) loop in a configured VLAN, resulting in a denial of service (DoS) condition for that VLAN. The vulnerability is due to a logic error when processing specific link-local IPv6 traffic. An attacker could exploit this vulnerability by sending a crafted IPv6 packet that would flow inbound through the wired interface of an affected device. A successful exploit could allow the attacker to cause traffic drops in the affected VLAN, thus triggering the DoS condition.

Affected products

• Cisco IOS XE: affected versions not specified; version 3.15.1xbs only; version 3.15.2xbs only; version 16.7.1 only; version 16.7.1a only; version 16.7.1b only; …

Published 2021-09-23. Last modified 2026-06-17.